Modern electronic equipment often includes bias circuitry for operating pairs of complementary output transistors. For instance, solid state audio frequency and operational amplifiers usually include a pair of complementary output transistors for driving an electrical load. A driver circuit is generally coupled from an input signal source to the complementary output transistors and a bias circuit is generally coupled to both the driver circuit and the complementary output transistors. In some instances, it is desirable for the foregoing circuitry to be manufactured in monolithic integrated form to facilitate low cost, minimum space and maximum reliability.
In general, it is desirable for such bias, driver and complementary output transistors to draw currents of minimum magnitudes from the power supply. By minimizing these current drains, the cost and size of the power supply is minimized and the amount of heat created is also minimized. By minimizing the heat generated efficiency is increased because additional power is not required to operate fans or additional heat sinking is not required to keep the system within the finite temperature range over which it will operate properly. It is well known that both active components and passive components must operate within a finite temperature range or such components are likely to fail. Semiconductor devices such as bipolar transistors are susceptible to thermal runaway ending in avalanche if the temperature thereof is not controlled. Moreover, extreme temperatures can cause vaporization of the metallization on the die and of the wire bonds which connect the die to the pinouts. Hence, it is desirable to minimize current drain and thus power dissipation whenever possible without undue sacrifice in performance.
The gain and frequency response of the driver circuit and hence of the amplifier are each proportional to the bias current of the driver circuit. Some of the early complementary transistor amplifier configurations required the bias circuit to be connected in series with the driver circuit. Thus, the bias circuit and hence the complementary output devices conducted a quiescent current which was a function of the large current required by the driver stage. As a result, the biasing devices and the complementary output devices conducted a larger current than necessary which resulted in them dissipating more heat than necessary.
To solve the above problem, a prior art monolithic integrated circuit implementation was devised wherein a separate bias current supply is provided to supply the majority of the current required by the driver circuit directly to the driver circuit while bypassing the complementary biasing devices. A parallel bias current for the complementary devices is also provided to the driver circuitry to maximize the gain and bandwidth of the amplifier. Generally, the foregoing parallel current sources are implemented in integrated circuit form by utilizing multiple collector PNP current supply transistors having emitters connected to the positive supply conductor of an integrated circuit. The base electrode is connected to the collector electrode of a PNP device in a known manner to complete the prior art current supply circuit.
Dynamic input signals of a negative polarity have a tendency to render the driver circuit of the improved prior art configuration nonconductive. In this condition, the output terminal of the driver circuit commonly used in such configurations tends to approach the positive supply potential. As a result, a positive potential is applied to one of the collector electrodes of the multiple collector PNP current supply transistor. This positive potential can result in forward biasing the base-to-collector junction of the current supply transistor and thus forcing it into saturation. The forward biased junction then provides base current otherwise provided by the emitter-to-base junction. As a result, the magnitude of the base drive current available to the current supply transistor is reduced. Consequently, all of the currents provided by the composite current supply transistor are undesirably reduced in magnitude. Thus, the base drive for the complementary NPN transistor is reduced so that not enough current can be supplied to load impedances having values less than a predetermined minimum value. As a result the output signal waveform is undesirably distorted.